The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
All publications identified herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
There is a long-standing need to efficiently and cost-effectively recover commercially valuable metals from low yield sources, such as mine tailings.
Historically, it has been especially desirable to recover alkaline earth elements. Alkaline earth elements, also known as beryllium group elements, include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium, (Ra), which range widely in abundance. Applications of these commercially important metals also vary widely, and include uses as dopants in electronic components, structural materials, and in the production foods and pharmaceuticals.
Methods of isolating of one member of the alkaline earth family, calcium, from minerals such as limestone, have been known since ancient times. In a typical process limestone is calcined or otherwise roasted to produce calcium oxide (CaO), or quicklime. This material can be reacted with water to produce calcium hydroxide (Ca(OH)2), or slaked lime. Calcium hydroxide, in turn, can be suspended in water and reacted with dissolved carbon dioxide (CO2) to form calcium carbonate (CaCO3), which has a variety of uses. Approaches that have been used to isolate other members of this family of elements often involve the production of insoluble hydroxides and oxides using elevated temperatures or strong acids. Such approaches, however, are not suitable for many sources of alkaline earth elements (such as steel slag), and are not sufficiently selective to be readily applied to mixtures of alkaline earth elements.
Hydrometallurgy can also been used to isolate metals from a variety of minerals, ores, and other sources. Typically, ore is crushed and pulverized to increase the surface area prior to exposure to the solution (also known as a lixiviant). Suitable lixiviants solubilize the desired metal, and leave behind undesirable contaminants. Following collection of the lixiviant, the metal can be recovered from the solution by various means, such as by electrodeposition or by precipitation from the solution.
Previously known methods of hydrometallurgy have several problems. Identification of lixiviants that provide efficient and selective extraction of the desired metal or metals has been a significant technical barrier to their adoption in the isolation of some metals. Similarly, the expense of lixiviant components, and difficulties in adapting such techniques to current production plants, has limited their use.
Approaches have been devised to address these issues. United States Patent Application No. 2004/0228783 (to Harris, Lakshmanan, and Sridhar) describes the use of magnesium salts (in addition to hydrochloric acid) as a component of a highly acidic lixiviant used for recovery of other metals from their oxides, with recovery of magnesium oxide from the spent lixiviant by treatment with peroxide. Such highly acidic and oxidative conditions, however, present numerous production and potential environmental hazards that limit their utility. In an approach disclosed in U.S. Pat. No. 5,939,034 (to Virnig and Michael), metals are solubilized in an ammoniacal thiosulfate solution and extracted into an immiscible organic phase containing guanidyl or quaternary amine compounds. Metals are then recovered from the organic phase by electroplating.
A similar approach is disclosed in U.S. Pat. No. 6,951,960 (to Perraud) in which metals are extracted from an aqueous phase into an organic phase that contains an amine chloride. The organic phase is then contacted with a chloride-free aqueous phase that extracts metal chlorides from the organic phase. Amines are then regenerated in the organic phase by exposure to aqueous hydrochloric acid. Application to alkaline earth elements (for example, calcium) is not clear, however, and the disclosed methods necessarily involve the use of expensive and potentially toxic organic solvents.
In a related approach, European Patent Application No. EP1309392 (to Kocherginsky and Grischenko) discloses a membrane-based method in which copper is initially complexed with ammonia or organic amines. The copper:ammonia complexes are captured in an organic phase contained within the pores of a porous membrane, and the copper is transferred to an extracting agent held on the opposing side of the membrane. Such an approach, however, requires the use of complex equipment, and processing capacity is necessarily limited by the available surface area of the membrane.
Methods for capturing CO2 could be used to recover alkaline earth metals, but to date no one has appreciated that such could be done. Kodama et al. (Energy 33 (2008), 776-784) discloses a method for CO2 capture using a calcium silicate (2CaO.SiO2) in association with ammonium chloride (NH4Cl). This reaction forms soluble calcium chloride (CaCl2), which is reacted with carbon dioxide (CO2) under alkaline conditions to form insoluble calcium carbonate (CaCO3) and release chloride ions (Cl−).
Kodama et al. uses clean forms of calcium to capture CO2, but is silent in regard to the use of other alkaline earth elements in this chemistry. That makes sense from Kodoma et al.'s disclosure, which notes that a high percentage (approximately 20%) of the NH4Cl used is lost in the disclosed process, requiring the use of additional equipment to capture ammonia vapor. This loss results in significant process inefficiencies, and raises environmental concerns. Japanese Patent Application No. 2005097072 (to Katsunori and Tateaki) discloses a similar method for CO2 capture, in which ammonium chloride (NH4Cl) is dissociated into ammonia gas (NH3) and hydrochloric acid (HCl), the HCl being utilized to generate calcium chloride (CaCl2) that is mixed with ammonium hydroxide (NH4OH) for CO2 capture.
International Application WO 2012/055750 (to Tavakkoli et al) discloses a method for purifying calcium carbonate (CaCO3), in which impure CaCO3 is converted to impure calcium oxide (CaO) by calcination. The resulting CaO is treated with ammonium chloride (NH4Cl) to produce calcium chloride (CaCl2), which is subsequently reacted with high purity carbon dioxide (CO2) to produce CaCO3 and NH4Cl, with CaCO3 being removed from the solution by crystallization with the aid of seed crystals. Without means for capturing or containing the ammonia gas that would result from such a process, however, it is not clear if the disclosed method can be adapted to an industrial scale.
Thus, there is still a need for a hydrometallurgical method that provides simple and economical isolation of metal hydroxide forming species.